There is relatively extensive knowledge available concerning ash transformation reactions during combustion of woody biomass. In recent decades, the use of these energy carriers has increased, from a low-technology residential small-scale level to an industrial scale. Along this evolution, ash chemical-related phenomena for woody biomass have been observed and studied. Therefore, presently the understanding for these are, if not complete, fairly good. However, because the demand for CO 2 -neutral energy resources has increased recently and will continue to increase in the foreseeable future, other biomasses, such as, for instance, agricultural crops, have become highly interesting. The ash-forming matter in agricultural biomass is rather different in comparison to woody biomass, with a higher content of phosphorus as a distinctive feature. The knowledge about the ash transformation behavior in these systems is far from complete. Here, an attempt to give a schematic but general description of the ash transformation reactions of biomass fuels is presented in terms of a conceptual model, with the intention to provide guidance in the understanding of ash matter behavior in the use of any biomass fuel, primarily from the knowledge of the concentrations of ash-forming elements. The model was organized in primary and secondary reactions. Restrictions on the theoretical model in terms of reactivity limitations and physical conditions of the conversion process were discussed and exemplified, and some principal differences between biomass ashes dominated by Si and P, separately, were outlined and discussed.
The bed agglomeration characteristics during combustion of phosphorus-rich biomass fuels and fuel mixtures were determined in a fluidized (quartz) bed reactor (5 kW). The fuels studied (separately and in mixtures) included logging residues, bark, willow, wheat straw, and phosphorus-rich fuels, like rapeseed meal (RM) and wheat distillers dried grain with solubles (DDGS). Phosphoric acid was used as a fuel additive. Bed material samples and agglomerates were studied by means of scanning electron microscopy (SEM) in combination with energy-dispersive X-ray spectroscopy (EDX), in order to analyze the morphological and compositional changes of coating/reaction layers and necks between agglomerated bed particles. Furthermore, bed ash particles were separated by sieving from the bed material samples and analyzed with SEM/EDS and powder X-ray diffraction (XRD). For logging residues, bark, and willow, with fuel ash rich in Ca and K but with low contents of P and organically bound Si, the bed layer formation is initiated by reactions of gaseous or liquid K compounds with the surface of the bed material grains, resulting in the formation of a potassium silicate melt. The last process is accompanied by the diffusion/dissolving of Ca into the melt and consequent viscous flow sintering and agglomeration. The addition of high enough phosphorus content to convert the available fuel ash basic oxides into phosphates reduced the amount of K available for the reaction with the quartz bed material grains, thus preventing the formation of an inner bed particle layer in the combustion of logging residues, bark, and willow. Some of the phosphate-rich ash particles, formed during the fuel conversion, adhered and reacted with the bed material grains to form noncontinuous phosphate-silicate coating layers, which were found responsible for the agglomeration process. Adding phosphorus-rich fuels/additives to fuels rich in K and Si (e.g., wheat straw) leads to the formation of alkali-rich phosphatesilicate ash particles that also adhered to the bed particles and caused agglomeration. The melting behavior of the bed particle layers/ coatings formed during combustion of phosphorus-rich fuels and fuel mixtures is an important controlling factor behind the agglomeration tendency of the fuel and is heavily dependent on the content of alkaline earth metals in the fuel. A general observation is that phosphorus is the controlling element in ash transformation reactions during biomass combustion in fluidized quartz beds because of the high stability of phosphate compounds.
The bed agglomeration characteristics during combustion of typical biomass fuels were determined in a bench-scale bubbling fluidized-bed reactor (5 kW) using olivine and quartz sand as bed material. The fuels studied include willow, logging residues, wheat straw, and wheat distiller’s dried grain with solubles (wheat DDGS). Bed material samples and agglomerates were analyzed by means of scanning electron microscopy coupled with energy-dispersive X-ray spectroscopy (SEM–EDS), for morphology and elemental composition. Furthermore, bed ash particles were separated by sieving from the bed material samples and analyzed for elemental composition by SEM–EDS and for determination of crystalline phases by powder X-ray diffraction (XRD). Chemical equilibrium calculations were performed to interpret the experimental findings of layer formation and reaction tendencies in both bed materials. Significant difference in the agglomeration tendency between olivine and quartz was found during combustion of willow and logging residues. These fuels resulted in inner layers that were more dependent on the bed material composition, and outer layers that have a composition similar to the fuel ash characteristics. The elemental composition of the inner layer formed on the quartz bed particles was dominated by Si, K, and Ca. In the olivine bed, the inner layer consisted mainly of Mg, Si, and Ca. Chemical equilibrium calculations made for both bed materials showed a low chemical driving force for K to react and be retained by the olivine bed particles, which is in accordance to the experimental findings. For the quartz case, the inner layer was found responsible for the initiation of the agglomeration process. The composition of the fewer and more porous agglomerates found after the experiments in the olivine bed showed neck composition and characteristics similar to the individual bed ash particles found in the bed or outer bed particle coating composition. For DDGS (rich in S, P, K, and Mg) and wheat straw (rich in Si and K), no significant differences in the bed agglomeration tendency between olivine and quartz bed materials were found. The results show that the bed particle layer formation and bed agglomeration process were associated to direct adhesion of bed particles by partly molten fuel ash derived K–Mg phosphates for DDGS and K-silicates for wheat straw.
A growing interest has been observed for the use of cereal grains in small-and medium-scale heating. Previous studies have been performed to determine the fuel quality of various cereal grains for combustion purposes. The present investigation was undertaken in order to elucidate the potential abatement of lowtemperature corrosion and deposits formation by using fuel additives (calcite and kaolin) during combustion of oat. Special emphasis was put on understanding the role of slag and bottom ash composition on the volatilization of species responsible for fouling and emission of fine particles and acid gases. The ash fractions were analyzed with scanning electron microscopy/energy dispersive spectroscopy (SEM/EDS), for elemental composition, and with X-ray diffraction (XRD) for identification of crystalline phases. The previously reported K and Si capturing effects of kaolin additive were observed also in the present study using P-rich biomass fuels. That is, the prerequisites for the formation of low melting K-rich silicates were reduced. The result of using kaolin additive on the bottom ash was that no slag was formed. The effect of the kaolin additive on the formation of submicrometer flue gas particles was an increased share of condensed K-phosphates at the expense of K-sulfate and KCl. The latter phase was almost completely absent in the particulate matter. Consequently, the levels of HCl and SO 2 in the flue gases increased somewhat. The addition of both calcite assortments increased the amount of formed slag, although to a considerably higher extent for the precipitated calcite. P was captured to a higher degree in the bottom ash, compared to the combustion of pure oat. The effect of the calcite additives on the fine particle emissions in the flue gases was that the share of K-phosphate decreased considerably, while the content of K-sulfate and KCl increased. Consequently, also the flue-gas levels of acidic HCl and SO 2 decreased. This implies that the low-temperature corrosion observed in small-scale combustion of oat possibly can be abated by employing calcite additives. Alternatively, if problems with slagging and deposition of corrosive matter at heat convection surfaces are to be avoided, kaolin additive can be utilized, on the condition that the higher concentrations of acidic gases can be tolerated.
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